Gas turbine engines contain a large variety of hollow structures. The temperatures and stresses under which most gas turbine engine components operate necessitate the fabrication of such structures from high strength, high temperature materials, such as titanium alloys and nickel-base or cobalt-base superalloys. These materials are expensive and generally very difficult and costly to machine; consequently, fabrication processes which can produce useful net, or near net, shapes are highly desired.
Airfoils mounted on rotating disks for use in gas turbine engines are usually subjected to high stresses and high temperatures during operation. Decreases in the weight of the airfoil can reduce operating stresses by reducing centrifugal force in these components, and can reduce the overall engine weight. Weight reduction is also important in stationary engine components, particularly for aircraft applications.
One potential technique for reducing weight is to use low density materials. Commonly available low density metals generally do not have properties suitable for withstanding temperatures and stresses encountered in the operation of these engines.
A more useful technique is to fabricate the airfoils from higher density heat resistant materials with hollow interiors, a technique which can provide internal cooling capability as well as weight reduction. This is commonly done by various techniques. For example, part halves can be cast or machined from solid stock with recesses on one or more mating surfaces such that when the halves are bonded together, components with hollow interiors are produced. Another technique builds up a hollow component from essentially flat sheets, which are then bonded together using either externally applied vacuum or internally applied pressure to conform the outer surfaces to a form die. Still another technique incorporates slicing of a solid component, hollowing out the interior, and rebonding the separated portions.
Another existing process utilizes an iron-base core prepared in the shape of the cavity desired in the hollow component. Superalloy or titanium alloy powder is forged around the iron-base core in a hot isostatic pressing (HIP) operation to form the desired article. The iron-base core is chemically removed to produce the hollow portion of the article. In addition to the high cost and waste disposal problems of this process, the surfaces of the cavity within the article can become contaminated This contamination must be removed in a subsequent operation.
With the importance of weight-reduction in aircraft propulsion applications, and the need to produce gas turbine engine components at minimum cost, it is highly desirable to develop methods for producing hollow objects from high-strength, high-temperature materials
Accordingly, it is an object of the invention to produce hollow superalloy or titanium alloy objects in a cost-effective manner. It is another object of the invention to fabricate hollow cores, with or without internal bracing, which provide a cavity and become part of a hollow object